On-line switching VFD supply - Commercial supply and back?

[artecerv]

I am considering using an arrangement that allow me to control the ramp up and ramp down of a conveyor belt with a VFD whilst using a DOL starter during normal operation.

I envisage the scheme to work in the following way:
• Starting up will be controlled by a VFD to provide a soft-jerk-free ramp up.
• Once start up is completed (i.e. ~ 50Hz), the on-line switching from VFD control to DOL control will be carried out.
• If stopping is required, the control circuit will switch the control back to the VFD control for ramping down.

The benefits sought with this scheme are:
• Reduce power dissipation and resultant heat. The dimensions of the switchroom limit the amount of heat that can be handled.
• Provide soft-starting and stopping in an application that demands high starting torque (client didn’t want to use fluid couplings).

Note: I realise that not all VFD are capable of doing the switch back to supply the motor. It will require some sort of synchronisation mechanism with the commercial supply (I have found a couple of brands whaich I believe are capable of doing what I need).

Have any of you implemented a scheme similar to this? Will I require switching transformers that might increase once again the size of my cabinets?

Can you please share your experiences with me?

Thanks in advance.

Regards,

 

[alexit]

Look at motor starters, exactly what you propose without VFD excess:

http://www.ab.com/en/epub/catalogs/12768/229240/229262/3073021/1203377/index.html

"Bulletin 150 — Reduced Voltage Starters"

 

[DickDV]

alexit, you missed an important detail. artecerv said that this is a high torque start.

When using a softstarter, the load limits how soft the start can be thru the breakaway torque level. An inverter will develop far more torque per amp at start than any softstarter except possibly a wound rotor motor/starter.

Since a fluid coupling is mentioned, it seems to me that the horsepower is not really small so that affects our choices too.

As much as I like inverters, I think that this just might be a good application for a wound rotor starter. An inverter will be troublesome because of the synchronous transfers required.

On the other hand, with drive heat losses only 2% of load kw these days, that's not really much heat. artecerv just might want to rethink his heat situation and just use an off-the-shelf solution like an inverter and keep it online even at full speed.

 

[jraef]

Closed Synchronous Transfer is a tricky business for any VFD, even the ones that are specialists in it such as Robicon. I would strongly suggest against trying to do it on your own, the consequences of failure are essentially the loss of the VFD. Also watch out for vendors who say they can do it, but use an open transition. Torque transients from doing that can destroy your mechanical components. If you have found someone other than Robicon that says they can do it, I would ask for a list of successful applications before I would accept their bids.

You may want to consider soft starters even in spite of DickDV's comment. He is correct in that soft starters main purpose is to reduce torque, so they may not work, but it all depends on what you mean by "high starting torque", that is a relative term. If a soft starter will work, it will cost far far less than a VFD. Many soft starter manufacturers will tell you up front if it will work if you can provide all the motor and load torque/speed curves as well as a detailed description of your power system and acceptable voltage drops etc.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[artecerv]

Hi DickDV,

I have to install 5 x 200kW 3Ø 690V AC VFDs in a transportable switchroom. Transportable switchroom dimensions are constrained by the size of roads and trucks (i.e. 5 meters width tops and 10 meters length).
(2% of 200kW) x 5 = 20kW of heat is considerable.

Kind Regards,

 

[DickDV]

I guess my suspicions of higher hp have been confirmed. And, yes, 20kw of heat is definitely considerable. jraef makes a good point--the high torque statement may not refer to breakaway torque but rather to high accel torque due to high inertia in the load.

The key to softstarting is breakaway torque. artecerv, could you please advise us on the nature of the load and, particularly, how much torque (roughly) it takes to get the load to move away from standstill.

 

[artecerv]

DickDV,

The loads are belt conveyors for a mine. The following data is extracted from the design data sheets:
Accel Torque= 140% of FLT (full load torque)
Bk/way Torque= ~90% of FLT (this is an average value)

Normally these values are too high for electronic soft starters (at least for the ones I have come across). I don't know of any site that uses electronic soft-starter for belt conveyors.

Note: full load torque is calculated using the motor synchronous speed and power rating.

Regards.

Jraef,

can you please provide contact details for Robicon? any particular person to talk to?

Thank you.

 

[CJCPE]

Perhaps eddy-current drives would be a viable alternative for this application. They would get most of the heat out of the switchroom to the motor location. They use magnetic couplings that have characteristics somewhat similar to fluid couplings.

For more info check:

http://www.dynamatic.com/

 

[jraef]

This really isn't a venue for providing names and phone numbers, but you can go to their website. The were recently bought by Siemens so their name has changed, but you can still find them at

http://www.asirobicon.com/

There are plenty of soft starters that can provide 90% starting torque and 140% acceleration torque (assuming a design B motor), but they are not the cheap ones. A good way to determine which ones will be capable is to look at thier overload capacity. If they are capable of at least 500% current for at least 30 seconds, they can deliver the torque you need. If rated for 350% current for 30 seconds, as many are, they cannot. Pay atention to the time issue as well because some claim to be capable of 550% curent, but if you look into it that is only for 2 seconds which will not be enough for your application. If they don't specify, that is a sign that they are on the weaker side of the spectrum.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Marke]

The very important thing to consider when using a reduced voltage starter for a high torque application, is that the motor determines the minimum current required to develop a particular torque. The correct selection of the motor is paramount to getting a "high start efficiency"
If you begin with a motor that produces low torque for high amps, you will not get a good result with anything other than an inverter. If you are careful and choose the right motor at the beginning, you will get good results.
I have been involved in many material conveyor applications with soft starters where good results have been achieved.
As an indicator of the relative starting effectiveness of induction motors, take the locked rotor torque (%) and divide this by the locked rotor current (%) and you wnat the highest result.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[LionelHutz]

How are you starting the conveyors right now? If you are using full-voltage starter then you can use a soft-starter. I've dealt with a number of customers who have successfully used soft-starters on conveyor belts.

The main requirement for full-voltage or soft-starter acceleration (of any load) is that the locked-rotor motor torque is greater than the worst case breakaway torque the conveyer requires. Generally speaking, the higher the difference between torques the better.

 

[artecerv]

LionelHutz,

The material handling system is for a Greenfield project(the system is being design from scratch).

Thanks all for your valuable comments.

Kind Regards,

 

[LionelHutz]

I wrote that wrong. The locked rotor torque > breakaway torque generally applies for conveyors. Usually, if you can make the conveyor move you're good.

The torque the motor produces is proportional to the square of the voltage (or current) reduction. For example, lower the voltage to the motor to 80% of rated and the torque is 64% (0.8^2) of rated. So, if you know your required load torque and the motor rated torque calculate sqrt(Tload/Tmotor) to give you the amount you could reduce the voltage or current. Calculate this at zero speed and you'll get an idea if soft-starters would be of any use. You do need a value greater than this minimum to actually get some accelerating torque but it gives you a quick idea of how much reduction is possible.

Of course, you didn't say if you wanted to use a VFD to take advantage of the long ramp-time it could provide. Some customers want to accelerate their long conveyors slowly to allow the pull on the belt to be slowly applied.

 

[Drivesrock]

Dear Artecerv,
the starting ramp of a long distance conveyor quite often is in two stages where the conveyor is initially accelerated to a low speed of, say, 10%, maintains this for a few seconds so that the rubber that stretched and caused the conveyor counter-weight to drop has time to go back to normal length and then the conveyor is accelerated upto working speed (in yr case 50Hz motor speed). This is especially important for long conveyors and when these are also started with material on them.
Stopping is normally a smooth "S" ramp back to zero speed with the VSD in control and the 'parking' brakes apply at zero speed.
Now, your VSD may require dynamic braking (with resistor bank) so that the conveyor stops via the ramp in the time you require it to stop.

I would be interested to know how your 5 drives/motors are mechanically arranged - all seperate or maybe two motors to one driving roll? This will influence what solutions are available to you.
Synch transfer is available from several VSD suppliers but I would only consider for this application those that offer parallel synch transfer as your conveyor will slow down very quickly if there is a time delay between drive off and DOL on and this will invariably cause a high current 'jolt' on the mains supply and on the conveyor mechanics itself.

Drivesrock